For some decades there has been searched for processes for the production of cellulose moulded bodies able to substitute the viscose process, today widely employed. As an alternative which is interesting for its reduced environmental impact among other reasons, it has been found to dissolve cellulose without derivatisation in an organic solvent and extrude from this solution moulded bodies, e.g. fibres, films and other moulded bodies. Fibres thus extruded have received by BISFA (The International Bureau for the Standardization of man made fibers) the generic name Lyocell. By an organic solvent, BISFA understands a mixture of an organic chemical and water.
It has turned out that as an organic solvent, a mixture of a tertiary amine-oxide and water is particularly appropriate for the production of cellulose moulded bodies. As the amine-oxide, primarily N-methylmorpholine-N-oxide (NMMO) is used. Other amine-oxides are described e.g. in EP-A-0 553 070. A process for the production of mouldable cellulose solutions is known e.g. from EP-A-0 356 419. For the purposes of the present specification and the present claims, the production of cellulose moulded bodies using tertiary amine-oxides generally is referred to as amine-oxide process.
In EP-A-0 356 419, an amine-oxide process for the production of spinnable cellulose solutions is described, wherein as a starting material among other substances a suspension of cellulose in liquid, aqueous N-methylmorpholine-N-oxide (NMMO) is used. This process consists in transforming the suspension in a thin-film treatment apparatus in one single step and continuously into a mouldable solution. Finally, the mouldable solution is spun into filaments by a forming tool such as a spinneret and the filaments are passed through a precipitation bath.
In the precipitation bath cellulose is precipitated. The tertiary amine-oxide is accumulated in the precipitation bath. The precipitation bath may contain up to 30 weight % of amine-oxide. For the economy of the amine-oxide process it is of vital importance to recover the amine-oxide as completely as possible and reuse it for the production of a mouldable cellulose solution. Thus it is necessary to recover NMMO from the precipitation bath.
In addition to the amine-oxide however, degradation products of the amine-oxide are also accumulated in the precipitation bath. These degradation products may be intensively coloured, thus deteriorating the quality of the cellulose moulded bodies produced. On the other hand, other substances may represent an additional safety risk, since under certain conditions the amine-oxide tends to show highly exothermic decomposition reactions and these decomposition reactions may be induced or accelerated by certain substances. These substances have to be removed from the precipitation bath which is to be regenerated before the NMMO is concentrated and separated in accordance with the purification process described in WO 97/07268.
After removing these unwanted substances, water is withdrawn from the purified precipitation bath which optionally is combined with other process liquids of the amine-oxide process such as vapour condensates formed during the production of the cellulose solution. This may be carried out for instance by means of evaporation. The residue of this evaporation contains highly concentrated aqueous amine-oxide which is recycled again into the amine-oxide process. The vapours of the evaporation consist mainly of water, wherein significant amounts of N-methylmorpholine, the main degradation product of NMMO, are also dissolved. Moreover, the vapours contain also NMMO and morpholine. Typically, the vapours contain up to 100 mg of NMMO, 240 mg of N-methylmorpholine and 30 mg of morpholine per liter. Conveniently, these vapours are concentrated, e.g., by means of reverse osmosis. The aqueous solution obtained contains typically up to 4 g of NMMO, up to 10 g of N-methylmorpholine and up to approximately 1 g of morpholine.
To keep the NMMO losses as low as possible, it is tried to reoxidize the N-methylmorpholine to NMMO. This may be achieved for instance by means of a peroxidic oxidant.
A process for the preparative production of tertiary amine-oxides by means of oxidation of tertiary amines is known e.g. from EP-A-0 092 862. According to this process, the amine-oxide is oxidized under pressure with molecular oxygen in an aqueous solvent, said solvent having a pH value approximately equal or higher than the pKa value of the tertiary amine.
DD-A-259 863 is concerned with the production of aqueous NMMO solutions by means of oxidation of N-methylmorpholine with H.sub.2 O.sub.2 and by passing the reaction solution over one or more exchanger columns filled with styrene/divinylbenzene copolymer containing sulphonate groups, as well as by adjusting a pH value of the solution to values ranging from 8 to 5 by addition of phosphoric acid.
In an oxidation it is disadvantageous that morpholine present in the process liquid introduced as a contamination together with the tertiary amines is partially transformed into toxic N-nitrosomorpholine, which is accumulated unwantedly in the NMMO cycle. Additionally, other nitrosoamines are also formed in the oxidation reactions.
Oxidation of N-methylmorpholine with H.sub.2 O.sub.2 to NMMO is known e.g. from EP-A-0 254 803. From DE-A-4 140 259, the production of NMMO by a process is known wherein the formation of nitrosoamines is restricted by scavenging primary and secondary amines, for instance by means of acid halides. EP-A-0 320 690 describes the production of amine-oxides substantially free from nitrosoamines by oxidation with peroxides in the presence of a combination of CO.sub.2 /ascorbic acid acting as a nitrosoamine inhibitor. From EP-A-0 401 503, oxidation with H.sub.2 O.sub.2 in water and a co-solvent, preferably a carboxylic acid ester, is known. According to FR-A-8 808 039, oxidation is carried out while adding CO.sub.2, and according to U.S. Pat. No. 5,216,154, oxidation to NMMO is carried out in a pure CO.sub.2 atmosphere.
In the state of the art, the forming of nitrosoamine either is not prohibited, or it is achieved by removing the starting products of the N-nitrosomorpholine or by employing additives to slow down the formation rate of the N-nitrosomorpholine. Particularly in an amine-oxide process comprising a closed cycle, the addition of various chemicals such as acid halides or ascorbic acid or CO.sub.2 to the process causes problems in the purification of the process liquids, since the degradation products introduced together with the added chemicals have to be removed from the process. For many chemicals, it is also necessary to consider safety aspects such as the risk of exothermic reactions. Thus, neither of the described processes is appropriate for the regeneration of process liquids of the amine-oxide process.